Dynamic regimes in the ac response ofYBa2Cu3O7

Abstract

We use ac susceptibility measurements to explore the dynamic response of vortices in YBCO single crystals pinned by aligned columnar defects at low dc fields. Using a combination of techniques for the analysis of the data, we investigate the vortex motion as a function of the amplitude and frequency of the applied ac field h ac , identifying the influence of both intra-and inter-valley motion. We build up a dynamic diagram in the h acT plane indicating the crossover lines among several regimes in the solid phase. At low h ac , a linear response with very low dissipation arises from the oscillation of pinned vortices inside the tracks ͑Campbell regime͒. At high h ac a critical state develops. The large influence of thermal fluctuations in this limit is indicated by the frequency dependence, which allows us to obtain the current density dependence of the activation energy and the glassy exponent. We also discuss the origin of the large nonlinear transition regime observed at intermediate amplitudes. ͓S0163-1829͑99͒02613-2͔

Unusually strong coherent response from grain-boundary Josephson network in polycrystalline Pr x Y1 − x Ba2Cu3O7 − δ

“…As it is evident from this picture, there are two distinctive regions. Namely, above h = 15Oe the curves exhibit almost linear dependence which can be attributed to establishment of the well-documented [18,19,20,21] Bean type critical state regime with 1+4πχ ′ B (h) = 2h/J C D where J C is the fieldindependent critical current density and D the sample's thickness. On the other hand, below h = 15Oe, practically temperature-independent periodic oscillations are clearly seen.…”

Crossover from intravalley to intervalley vortex motion in type-II superconductors with a periodic pinning array

“…In this regime the initial vortex configuration is strongly disturbed after the ac measurement. 7,12 In a recent work, we have shown that it is precisely this invasive character of the measurements in the critical state ͑CS͒ which allows one to determine the critical current density J. 8 In this work, using the ac-susceptibility technique we identify the intravalley and intervalley motion of vortices by several different approaches, all of them showing good degree of consistency.…”

“…7 By using the criterion onset Љ = 0.01, the ͑h͒ curves allow one to determine the onset h * ͑T͒ of nonlinearity. 8 The resulting boundaries h * ͑T͒ for the plain Pb film and for the antidot sample at H = 5 Oe are shown as open symbols in Figs.…”

“…10,11 In this so-called Campbell regime, it is possible to determine the average curvature of the pinning potential ␣ L assuming a parabolic potential well. 6,7 For higher amplitudes, the average distance traveled by vortices as they follow the ac drive approaches the pinning range and a nonlinear response is observed. Further increasing the acamplitude vortices overcomes the pinning potential well and a critical state, characterized by a higher dissipation and a penetration depth proportional to the ac field amplitude, develops.…”

Dynamic regimes in films with a periodic array of antidots

“…[12] In a previous work [2] we have determined for a similar sample, that λ(t = 0.90) ∼ 250 nm and using the J(T ) from figure 3, we obtain a crossover field h cs ∼ 40 G, which turns out to be far above our experimental estimation of h cs ∼ 1 G according to figure 3. Recently, Pasquini et al [13] have pointed out that in the case of transverse geometry, since the actual range of field penetration does not coincide with the penetration depth, the previous criterion should be modified. Assuming that the range of field penetration in both regimes coincides when |χ ′ | = 0.5, the authors show that the crossover should occur at h cs ∼ 9δ/2Rλ(T )J(T ), where R is the radius of a disk shaped sample.…”